The present invention relates to a catalyst, primarily for decomposing methanol to carbon monoxide and hydrogen. The invention also relates to application of this catalyst for decomposing methanol.
Catalytic decomposition of methanol for obtaining a hydrogen-rich gas has become of special interest in connection with use of methanol as a fuel for internal combustion engines and combustion turbines. Further, decomposition of methanol for the production of very pure hydrogen for hydrogen consuming processes is an alternative for conventional hydrogen producing processes.
U.S. Pat. No. 4,185,456 describes a method for the use of at least partly decomposed methanol as a fuel for combustion turbines. The heat of vaporization and decomposition of methanol is taken from the exhaust gas from the combustion. As stated in that patent, superheating and dissociating methanol to carbon monoxide and hydrogen, increases the energy content of approximately 20%. For combustion turbines, the energy content in the exhaust gas is more than enough to superheat the methanol and dissociate it. The method makes it possible both to supply the power for the turbine and to produce a valuable process gas containing hydrogen and carbon monoxide. The catalyst for the dissociation of methanol is selected from catalysts for the reverse reaction of forming methanol from carbon monoxide and hydrogen.
Application of dissociated methanol for internal combustion engines is described in a paper by J. Finegold et al. presented at the 1982 World Hydrogen Energy Conference IV, Pasadena, Calif., USA. The concept was demonstrated by mounting a reactor into the engine compartment of a 1980 Chevrolet Citation such that the system could be road-tested. The catalyst used was a copper-zinc oxide catalyst supported on alumina pellets. The heat for vaporizing methanol was taken from the engine coolant, while the heat for dissociating methanol was taken from the exhaust gas. A substantial improvement of brake thermal efficiency compared to gasoline systems was demonstrated. At low load the exhaust temperature and heat content was not sufficient to complete dissociation of all the methanol with the catalyst used. This means that some of the chemical energy gain is lost, but sufficient dissociation to gain the advantages due to lean burning was always obtained.
It is well known that catalysts based on copper and zinc oxide are active for synthesis of methanol from carbon monoxide and hydrogen. In addition these catalysts contain a trivalent metal oxide such as chromium or aluminium oxide. Such catalysts are described in DE Nos. 28 46 614 and 30 46 840. Generally speaking, catalytic decomposition of methanol can be achieved with the same type of catalyst as the synthesis of methanol. These catalysts are also active for steamreforming of methanol. However, the drawback for most of the copper-zinc-oxide catalysts is that they tend to be mechanically weak, and the mechanical properties of the catalyst are very important when it is to be used in a vehicle. Thus a conventional methanol synthesis catalyst would hardly be suitable in vehicles.
Another problem in catalytic decomposition of methanol is that several competing reactions may take place, and not all of these reactions are desired. Thus it is most important that the catalysts has a high selectivity for the most desired reaction. The following reactions may take place: EQU 1. CH.sub.3 OH.revreaction.CO+2H.sub.2 EQU 2. CO+3H.sub.2 .revreaction.CH.sub.4 +H.sub.2 O EQU 3. CO+H.sub.2 O.revreaction.H.sub.2 +CO.sub.2 EQU 4. CH.sub.3 OH+H.sub.2 O.revreaction.3H.sub.2 +CO.sub.2 EQU 5. 2CH.sub.3 OH.revreaction.CH.sub.3 OCH.sub.3 +H.sub.2 O
The catalyst should be most selective for reaction No. 1. When methanol dehydrates to form dimethyl ether (DME) and water, reaction No. 5, the energy content is increased considerably less than when methanol decomposes according to reaction No. 1. Thus reaction No. 5 is indeed undesired when methanol is used for fuel purposes. Reactions 2 and 3 occur to a very low extent for catalysts containing copper. Reactions 3 and 4 will only occur if reaction 5 takes place.